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BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20190205T113000
DTEND;TZID=Europe/Paris:20190205T123000
DTSTAMP:20260422T065025
CREATED:20190122T103242Z
LAST-MODIFIED:20190416T084247Z
UID:4078-1549366200-1549369800@lmv.math.cnrs.fr
SUMMARY:AG : Béatrice Chetard (Univ. Strasbourg & Univ. Western Ontario\, Canada)
DESCRIPTION:L’anneau des caractères (complexes) d’un groupe G est équipé d’une filtration dite de Grothendieck\, induite par les puissances extérieures des représentations de G. Peut-on calculer explicitement l’anneau gradué associé à cette filtration ? Quels liens y a-t-il entre l’anneau gradué de G et celui de ses sous-groupes ?
URL:https://lmv.math.cnrs.fr/evenenement/beatrice-chetard-univ-strasbourg-univ-western-ontario-canada/
LOCATION:Bâtiment Fermat\, salle 2205\, 45 avenue des Etats-Unis\, Versailles\, 78000\, France
CATEGORIES:Séminaire AG
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20190205T113000
DTEND;TZID=Europe/Paris:20190205T123000
DTSTAMP:20260422T065025
CREATED:20190212T154833Z
LAST-MODIFIED:20190416T122415Z
UID:4197-1549366200-1549369800@lmv.math.cnrs.fr
SUMMARY:PS : Michael Wallner (Université de Bordeaux) : Periodic Pólya Urns and Asymptotics of Triangular Young tableaux
DESCRIPTION:Pólya urns are urns where at each unit of time a ball is drawn uniformly at random and is replaced by some other balls according to its colour. We introduce a more general model : The replacement rule depends on the colour of the drawn ball and the value of the time mod p.\nOur key tool are generating functions\, which encode all possible urn compositions after a certain number of steps. The evolution of the urn is then translated into a system of differential equations and we prove that the moment generating functions are D-finite. From these we derive asymptotic forms of the moments. When the time goes to infinity\, we show that these periodic Pólya urns follow a rich variety of behaviours : their asymptotic fluctuations are described by a family of distributions\, the generalized Gamma distributions\, which can also be seen as powers of Gamma distributions.\nFurthermore\, we establish some enumerative links with other combinatorial objects\, and we give an application for a new result on the asymptotics of Young tableaux : This approach allows us to prove that the law of the lower right corner in a triangular Young tableau follows asymptotically a product of generalized Gamma distributions. This is joint work with Cyril Banderier and Philippe Marchal.
URL:https://lmv.math.cnrs.fr/evenenement/michael-wallner-universite-de-bordeaux-periodic-polya-urns-and-asymptotics-of-triangular-young-tableaux/
LOCATION:Bâtiment Fermat\, salle 2107
CATEGORIES:Séminaire PS
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BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20190207T140000
DTEND;TZID=Europe/Paris:20190207T150000
DTSTAMP:20260422T065025
CREATED:20190212T153117Z
LAST-MODIFIED:20190913T141330Z
UID:4190-1549548000-1549551600@lmv.math.cnrs.fr
SUMMARY:EDP : Patrick Ciarlet (ENSTA) : How to solve problems with sign-changing coefficients
DESCRIPTION:This talk summarizes joint works by the speaker and Anne-Sophie Bonnet-Ben\nDhia\, Lucas Chesnel\, Camille Carvalho and Juan-Pablo Borthagaray on how to solve problems with discontinuous\, sign-changing coefficients. \nIn electromagnetic theory\, the effective response of specifically designed materials is modeled by strictly negative coefficients: these are the so-called negative materials. Transmission problems with discontinuous\, sign-changing coefficients then occur in the presence of negative materials surrounded by classical materials. For general geometries\, establishing Fredholmness of these transmission problems is well-understood thanks to the T-coercivity approach.\nLet σ be a parameter that is strictly positive in some part of the computational domain\, and strictly negative elsewhere. We focus on the scalar source problem: find u such that divσ∇u − ω^2 u = f plus boundary condition\, where f is some data and ω is the pulsation. Denoting by σ+ the strictly positive value\, and by σ− the strictly negative value\, one can prove that there exists a critical interval\, such that the scalar source problem is well-posed in the Fredholm sense if\, and only\, if\, the ratio σ−/σ+ lies outside the critical interval. One may derive similar results for the related eigenvalue problem.\nWhen the ratio σ−/σ+ lies outside the critical interval\, the shape of the interface separating the two materials must be taken into account to solve the problems numerically. We propose a treatment which allows to design meshing rules for an arbitrary polygonal interface and then recover standard error estimates.\nThis treatment relies on the use of simple geometrical transforms to define the meshes.\nIn a last part\, we discuss the case where the ratio σ−/σ+ lies inside the critical interval.
URL:https://lmv.math.cnrs.fr/evenenement/patrick-ciarlet-ensta-how-to-solve-problems-with-sign-changing-coefficients/
LOCATION:Bâtiment Sophie Germain\, salle G210
CATEGORIES:Séminaire EDP
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20190212T113000
DTEND;TZID=Europe/Paris:20190212T123000
DTSTAMP:20260422T065025
CREATED:20190212T155018Z
LAST-MODIFIED:20190416T122544Z
UID:4199-1549971000-1549974600@lmv.math.cnrs.fr
SUMMARY:PS : Charles Tillier (Telecom ParisTech) : Estimation in semi-parametric transformation boundary regression models
DESCRIPTION:Boundary regression models naturally arise in many applications for instance when analysing auctions or records but also in production frontiers and image\nanalysis. Before fitting a regression model it is very common to transform the response variable to gain effciency in the statistical inference. In this talk\, we will consider parametric transformations that induce independence of the error distribution from the points of measurements. In such a context\, if the transformation of the response is monotone\, the attractive feature is that one may recover the original functional dependence in an easy manner. The main purpose of this talk is to investigate the consistency of an estimator based on a minimum distance approach in the context of nonparametric regression models with one-sided errors. In particular\, we estimate the transformation parameter\nand give mild model assumptions under which the estimator is consistent\, for both random covariates and fixed design points. The small sample behavior will be shown in a simulation study using the so-called Yeo-Johnson transformations.
URL:https://lmv.math.cnrs.fr/evenenement/charles-tillier-telecom-paristech-estimation-in-semi-parametric-transformation-boundary-regression-models/
LOCATION:Bâtiment Fermat\, salle 2107
CATEGORIES:Séminaire PS
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20190214T140000
DTEND;TZID=Europe/Paris:20190214T150000
DTSTAMP:20260422T065025
CREATED:20190212T153517Z
LAST-MODIFIED:20190415T141922Z
UID:4192-1550152800-1550156400@lmv.math.cnrs.fr
SUMMARY:EDP : Aris Daniilidis (CMM\, University of Chile \, Santiago) : Lipschitz functions that saturate their Clarke subdifferential.
DESCRIPTION:We prove that the set of Lipschitz functions with the property of having a maximal Clarke subdifferential at every point contains a linear subspace of uncountable dimension. Our approach is constructive. In particular\, in strong contrast to a previous result of similar flavour\, by J. Borwein and X. Wang\, we do not make use of the Baire category theorem. \nIn particular we establish lineability (and spaceability for the Lipschitz norm) of the above set inside the set of all Lipschitz continuous functions. (Joint work with G. Flores\, University of Chile)
URL:https://lmv.math.cnrs.fr/evenenement/aris-daniilidis-cmm-university-of-chile-santiago-lipschitz-functions-that-saturate-their-clarke-subdifferential/
LOCATION:Bâtiment Sophie Germain\, salle G210
CATEGORIES:Séminaire EDP
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20190219T113000
DTEND;TZID=Europe/Paris:20190219T123000
DTSTAMP:20260422T065025
CREATED:20190212T151554Z
LAST-MODIFIED:20190416T084449Z
UID:4186-1550575800-1550579400@lmv.math.cnrs.fr
SUMMARY:AG : Mattia Galeotti  (Univ. di Trento) : Singularities and global geometry of moduli of curves
DESCRIPTION:In a series of recent papers\, Chiodo\, Farkas and Ludwig carried out a deep analysis of the singular locus of the moduli space of stable (twisted) curves with an $\ell$-torsion line bundle. This opens the way to a computation of the Kodaira dimension without desingularizing\, as done by Farkas and Ludwig for $\ell=2$\, and by Chiodo\, Eisenbud\, Farkas and Schreyer for $\ell=3$. \nWe can generalize this works in two directions. At first we treat roots of line bundles on the universal curve systematically:  we consider the moduli space of curves C with a line bundle $L$ such that $L^{\xx\ell}\cong\omega_C^{\xx k}$. \nNew loci of canonical and non-canonical singularities appear for any $k\not\in\ell\Z$ and $\ell>2$\, we provide a set of combinatorial tools allowing us\nto completely describe the singular locus in terms of dual graphs. Furthermore\, we treat moduli spaces of curves with a $G$-cover where $G$ is any finite group.\nIn particular for $G=S_3$ we can evaluate the Kodaira dimension of the moduli space.
URL:https://lmv.math.cnrs.fr/evenenement/ronan-terpereau-inb-dijon/
LOCATION:Bâtiment Fermat\, salle 2205\, 45 avenue des Etats-Unis\, Versailles\, 78000\, France
CATEGORIES:Séminaire AG
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20190219T113000
DTEND;TZID=Europe/Paris:20190219T123000
DTSTAMP:20260422T065025
CREATED:20190212T155159Z
LAST-MODIFIED:20190918T073756Z
UID:4201-1550575800-1550579400@lmv.math.cnrs.fr
SUMMARY:PS : Adrien Mazoyer (UQAM\, Université du Québec à Montréal) : Modèles de mutation : étude probabiliste et estimation paramétrique
DESCRIPTION:Les modèles de mutations décrivent le processus d’apparitions rares et aléatoires de mutations au cours de la croissance d’une population de cellules. Les échantillons obtenus sont constitués de nombres finaux de cellules mutantes\, qui peuvent être couplés avec des nombres totaux de cellules ou un nombre moyen de cellules en fin d’expérience. Le modèle classique\, dit de Luria-Delbrück\, suppose que les développements cellulaires des cellules s’effectuent selon un processus de Yule. On peut dans ce cas expliciter la loi du nombre final de mutantes. Elle dépend de deux paramètres\, qui sont le nombre moyen de mutations et le paramètre de fitness (rapport des taux de croissance des deux types de cellules). Le problème statistique consiste à estimer ces paramètres au vu d’un échantillon de nombres finaux de mutantes. Diviser l’estimation du nombre moyen de mutations par le nombre total de cellules permet alors d’estimer la probabilité d’apparition d’une mutation au cours d’une division cellulaire qui est le véritable paramètre d’intérêt. L’estimation de cette probabilité est d’une importance cruciale dans plusieurs domaines de la médecine et de biologie : rechute de cancer\, résistance aux antibiotiques de Mycobacterium Tuberculosis\, etc. Cependant\, les hypothèses de modélisation classiques sont irréalistes : durées de vie exponentielles\, indépendance\, taille finale de la population constante\, absence de mort cellulaire… Il est donc nécessaire de disposer de méthodes d’estimation robustes pour lesquelles le biais\, en particulier sur la probabilité de mutation\, reste le moins sensible possible aux hypothèses de modélisation. Dans cet exposé\, nous présenterons un modèle de mutations permettant de considérer des processus de croissance inhomogènes en temps\, tout en généralisant les extensions déjà étudiées. Le problème statistique sera également traité: différentes méthodes d’estimation seront exposées\, et quelques sources de biais seront illustrées à l’aide d’études de simulation. Tous les résultats présents dans cet exposé ont par ailleurs été implémentés sous forme d’un package R qui sera brièvement présenté.
URL:https://lmv.math.cnrs.fr/evenenement/adrien-mazoyer-uqam-universite-du-quebec-a-montreal-modeles-de-mutation-etude-probabiliste-et-estimation-parametrique/
LOCATION:Bâtiment Fermat\, salle 2107
CATEGORIES:Séminaire PS
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20190225T110000
DTEND;TZID=Europe/Paris:20190225T120000
DTSTAMP:20260422T065025
CREATED:20190212T155409Z
LAST-MODIFIED:20190416T134516Z
UID:4203-1551092400-1551096000@lmv.math.cnrs.fr
SUMMARY:CRYPTO : Albrecht Petzoldt (LMV) : Cryptanalysis of the HiMQ-3 Signature Scheme
DESCRIPTION:The HiMQ-3 signature scheme is a very efficient multivariate signature scheme and was one of the first round candidates of the NIST post-quantum standardization process. \nIn this talk we present a new attack against HiMQ-3\, the so called singularity attack. The attack uses a large number of (message/signature) pairs to recover an equivalent private key\, which allows an attacker to forge signatures for arbitrary messages. \nWhile our attack does not break the scheme completely\, it shows that the security claims of the authors are not correct.
URL:https://lmv.math.cnrs.fr/evenenement/albrecht-petzoldt-lmv-cryptanalysis-of-the-himq-3-signature-scheme/
LOCATION:Bâtiment Descartes\, salle 301
CATEGORIES:Séminaire CRYPTO
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